Items on the marketplace count towards the "total items" on a character for the purposes of muling. For instance, if you have 8 slot, 6 items in your inventory, and 2 items on the market, your total item count on that character is 8, which is 8/8, full. This may be changed at a future date, but it is intentionally designed this way for the time being, to prevent the marketplace acting as a secondary buffer of muled items.